In economics, “we can model utility as logarithmic in wealth”, even after adding human capital to wealth, feels like a silly asymptotic approximation that obviously breaks down in the other direction as wealth goes to zero and modeled utility to negative infinity.
In cosmology, though, the difference between “humanity only gets a millionth of its light cone” and “humanity goes extinct” actually does feel bigger than the difference between “humanity only gets a millionth of its light cone” and “humanity gets a fifth of its light cone”; not infinitely bigger, but a lot more than you’d expect by modeling marginal utility as a constant as wealth goes to zero.
This is all subjective; others’ feelings may differ.
(I’m also open in theory to valuing an appropriately-complete successor to humanity equally to humanity 1.0, whether the successor is carbon or silicon or whatever, but I don’t see how “appropriately-complete” is likely so I’m ignoring the possibility above.)
The “understandable”+”exploit” category would include my personal favorite introduction, the experiment in Chapter 17. From “Thursday” to “that had been the scariest experimental result in the entire history of science” is about 900 words. This section is especially great because it does the whole “deconstruction of canon”/”reconstruction of canon” bit in one self-contained section; that pattern is one the best aspects of HPMOR but usually the setup and the payoff are dozens of chapters apart, with many so interleaved with the plot that the payoff counts as a major spoiler.
On the other hand, that section works best if you already know what P and NP and RSA cryptography are (and if you’re somehow still not too nerdy to wince at Harry’s lazy way of talking about the former); know your audience.
My kids’ favorite section is the introduction to Draco in Madam Malkin’s in Chapter 5, which IMO qualifies as “understandable”+”gobsmacking”, but it’s a bit long for you, 1500 words from “You are to get fitted for your robes, nothing else” to “poor Madam Malkin and her two poor assistants.”, and I don’t see any way to reasonably cut out a third of that without wrecking the humor.